1. Field of the Invention
The present invention relates to a transmitting power control unit, having a wide-band which is utilized in mobile communication equipment, such as automobile telephones and portable telephones.
2. Description of Related Art
FIG. 1 is a block diagram of a conventional transmitting power control unit. In FIG. 1, a reference numeral 11 represents a control section for generating channel information for designating a transmission frequency and power control information for designating the power to be transmitted, 12 represents a PLL circuit for stabilizing the transmission frequency, 13 represents a power amplifier for amplifying the output of the PLL circuit 12, 14 represents a directional coupler for detecting the passing power, 15 represents a filter for removing higher harmonic components of a transmission signal to be transmitted and also for suppressing thermal noise, 16 represents an antenna for radiating a transmission signal in the air, 17 represents a detector diode for detecting a signal obtained by the directional coupler 14, 18 represents a differential amplifier for amplifying the difference between a detected signal voltage detected by the detector diode 17 and a fixed reference voltage, an output of differential amplifier 18 being supplied to the power amplifier 13 as a power control signal, and 19 represents a fixed voltage generator for generating the fixed voltage. Namely, an Auto Power Control (APC) circuit for controlling a power of a transmission signal is constituted by power detecting means composed of the power amplifier 13, the directional coupler 14, and the detector diode 17 and the differential amplifier 18 receiving the detected voltage obtained by the power detecting means and the above-mentioned fixed voltage as input.
Next, the operation of the above-mentioned conventional example will be described. In FIG. 1, a transmission signal having a prescribed frequency band is generated from the PLL circuit 12 in accordance with channel information from the control section 11. This transmission signal is radiated in the air from the antenna 16, through the directional coupler 14 and the filter 15, after being amplified by the power amplifier 13.
Further, a prescribed fixed voltage is generated by the fixed voltage generator 19 in accordance with power control information from the control section 11, and supplied to one input terminal of the differential amplifier 18. A signal obtained by the directional coupler 14 is detected by the detector diode 17 and supplied to the other input terminal of the differential amplifier 18.
Thus, the output power from the power amplifier 13 is controlled by the power control signal outputted from the differential amplifier 18 so as to correspond to the above-mentioned prescribed fixed voltage.
In the conventional transmitting power control unit described above, however, there is a problem that attenuation quantity differ between e.g. the output level for L channel having a low frequency and H channel having a high frequency due to attenuation characteristics of the filter 15, as shown in FIG. 2 in the transmission band. In other words, the output level is as high as L.sub.L in the L channel, and the output level is as low as L.sub.H in the H channel of the transmission band. Thus, the output level changes almost linearly between both channels, and the output level differs depending on the transmission frequency band.
Further, a proposal has also been made that the fixed voltage be supplied to the differential amplifier after being amplified by a voltage-controlled amplifier, and using the control voltage of the voltage-controlled oscillator for generating a transmission signal also as a control signal of the voltage-controlled amplifier, so that the output voltage of this voltage-controlled amplifier is varied by the frequency of the transmission signal, and the output of the transmitter is compensated using the above, to thereby obtain a constant output in a wide-band (Japanese Patent Application, JP-A- 1-293016, published Nov. 27, 1989 entitled "CIRCUIT FOR CONTROLLING OUTPUT OF TRANSMITTER"). According to this method, however, such a problem is involved that a voltage-controlled amplifier is required excessively.